Follow path constraint - object goes back and forth when speeding
Instead of using a follow path constraint you can do this:
Add an empty in the world origin. Parent your sphere to it (Keep offset). Then animate the z-Rotation of the empty/sphere, which is much easier to use.
I've taken your blend file and parented the sphere to a rotating empty which is parented to an empty that's moving back and forth on the Y axis. The elliptical curve object is just to visualize the path of the sphere.
Your original sphere had an orbit with a diameter of 33.8 metres, so I set the sphere 16.9 metres away from the empty which makes a full rotation in 201 frames (to make it loop correctly on 200 frames). The second empty moves from Y = 3 m to Y = -3 m on frame 101 and back to Y = 3 m on frame 201.
This results in an elliptical movement which still has a diameter of 33.8 m on the major axis on the X axis, but the minor axis along Y is reduced by 6 m to 27.8, the Y distance that the second empty travels from one side to the other.
Because the rotating empty is 3 m away from the oscillating empty on the Y axis, the center of the elliptical movement is at X = 0 and Y = -3. (By the way, I've set the movement's interpolation to Bézier while the rotation is set to Linear, because otherwise the ellipse is somehow flattened on the short ends.)
This is because on frame 1 the sphere is on its lowest Y position and the osciallting empty is moving in negative Y direction at the beginning 8towards the sphere). If the sphere was on it highest Y position and the oscillating empty starts moving away from it, the ellipse diameter would be stretched in Y direction, exactly + 6 m.
If you start with the sphere in its lowest or highest X location, this will result in a slightly angled ellipse with different diameters (which can surely be calculated by formulas, but I don't have them at hand right now).